The need to remove all or selected species of ions from liquids is a common problem in many industries. One fast growing market is the desalination of brackish and sea water for municipal water supplies. Typical ways to separate the dissolved salts from water include phase change technologies, such as flash distillation, or membrane-based technologies such as reverse osmosis, both of which are energy intensive. For brackish water, two alternative techniques that use transverse electric fields are available: electrodialysis (ED) and capacitive de-ionization (CD). However, both of these techniques have a number of drawbacks.
In ED, a feed stream flows between two parallel membranes. An applied voltage perpendicular to the flow causes the dissolved ions to move, with the negatively charged ions (e.g., chloride) traveling towards the anode and the positively charged species (e.g., sodium) towards the cathode. The membranes are designed to let the corresponding ions leave the feed stream, but not to reverse direction. This causes the feed stream to de-ionize, while the waste streams in the adjacent compartments increase in salinity. A pair of feed and waste stream plus the membranes between them forms a cell. The main problem with ED techniques is that the voltage drop across the feed and waste channels varies with changes in ion concentration in the different channels and along the channels. This leads to energy inefficiencies as the optimal electric field condition can only be maintained across a small section of the entire system.
In CD, a process stream flows between two parallel electrodes designed to have a very large surface area. An applied voltage perpendicular to the flow causes the dissolved ions to move. The porous electrodes capture the ions in a Debye layer which can hold a large number of ions due to the large surface area. Once the electrode surface is saturated, a regeneration step starts where the voltage is turned off, or a small reverse voltage is applied to repel the ions back into the channel. This water, which can have a much higher ion concentration than the feed water is discarded and the whole process is repeated. The problem with CD is that it is a non-continuous process and relies on the long-term reversibility of the ion trapping and release mechanism.